In wide-band code division multiple access (W-CDMA) systems, precise power control is one of the basic requirements for high system capacity. Transmit powers in the down-link should be kept as low as possible in order to minimise interference, but high enough to ensure the required quality of service. Furthermore, in the downlink, it is desirable to provide a marginal amount of additional power to mobile stations at the cell edge, as they suffer from increased other-cell interference.
Even though a relatively slow power control algorithm is able to compensate for large-scale attenuation, distance attenuation and shadow fading, a fast power control algorithm is needed for multi-path fading for slowly moving mobiles.
In the forward link, i.e. the down-link, of a wide-band code division multiple access (WCDMA) system, the inner-loop power control adjusts the base-station transmit power in order to keep the received down-link signal-to-interference ratio (SIR) at a given target level (at the mobile station). The SIR target is defined according to the quality requirements. For reliable SIR estimation, narrowband estimation (after de-spreading) has to be employed. This is important especially in the down-link direction of the W-CDMA system because of the usage of orthogonal channelization codes. Tight delay requirements of the power control demand that the SIR estimation has to be obtained on a slot-by-slot basis.
The SIR estimation is divided into: a) signal power estimation; and b) interference power estimation.
In the forward link of a wide-band code division multiple access (WCDMA) system, a primary common pilot channel (P-CPICH) is broadcast over the entire cell or a sector. The P-CPICH is broadcast also in the case of a multi-beam arrangement (multiple beams per sector) and in the user specific beamforming. Therefore there always exists one such channel per sector regardless of the applied transmission scheme.
In the case of single antenna transmission, the interference power estimation is currently performed based on the primary common pilot channel. However, when utilising adaptive antenna techniques the dedicated channels are usually transmitted through a narrow beam, which means that the P-CPICH and down-link physical channels (DL-DPCH) experience different channel characteristics in transmission to the mobile station antenna. Due to that the fading of DL-DPCH can be almost uncorrelated compared to that of P-CPICH. The correlation depends on the angular spread of the radio channel (seen from the base transceiver station).
In current known systems, the down-link dedicated physical control channel (DL-DPCCH) is used for power interference estimation for the DL-DPCH (beam signal) in adaptive antenna systems because of the fact that the P-CPICH (antenna signal) does not usually experience the same channel characteristics on transmission to a mobile station antenna.
It is an aim of the present invention to provide an improved technique for estimating interference power in the receiver of an adaptive antenna system.